One of the necessary and important criteria for producing an acceptable polyester film product for a wide-format color plotter is a film that will accurately reproduce colors. While the water-solvent ink reproduces the desired colors on a coated paper product, good color reproduction for a film product has generally been unachievable.
It is well-known to those skilled in the art that the ink used in the popular, wide-format color plotter contains high levels of glycol. This is an advantage when printing on paper, because the ink will not cause paper deformation. However, on film, the density is low; the surface becomes tacky; drying is inordinately long; and the colors tend to bleed with time.
U.S. Pat. No. 5,190,805 discloses a composition containing pigments for producing an adequate ink jet layer material coated on a film base. The composition contains an acrylic comb polymer, which can be obtained from Soken Chemical Company of Japan. The polymer is described as having an acrylic backbone with grafted side chains of 2-hydroxy ethyl methylmethacrylate. The comb polymer, which is usually water-insoluble, is combined with a water-soluble resin. This causes the composition to become water-swellable (i.e., able to absorb water-based inks). However, when glycol inks are ink-jetted upon this material layer, the color performance is seriously degraded. The layer produces a soft and "greasy"-feeling print, which is commercially unacceptable. Over 24 hours are required for the glycol to finally evaporate from this material, wherein the print surface becomes sufficiently hard to allow handling.
It is generally impossible to prevent glycol from attacking the film base of the aforementioned article. Therefore, a top coat is usually employed to control glycol penetration and allow for glycol evaporation, so that the integrity of the film base will be preserved. The top coat also has the advantage of controlling the dot size during ink jet printing. Water-soluble or water-sensitive polymers are a preferred choice of materials for the top coat. The polymers must have permeability for the glycol, but should not be soluble in glycol. This criterion limits the available choices of cross-linkable polymers.
A curious effect occurs, however, when using a top coat. Any water of hydration remaining in the top layer will, in time, diffuse into the film base. This causes the top layer to become more resistant to ink penetration. A subsequent loss of density or dot growth is observed. The addition of humectants to the top layer will not cure this condition, because they, too, will eventually be drawn into the film base.
The present invention features a film article and method of making same, wherein the difficulties of producing an adequate film for a glycol ink jet device are addressed. The film, having almost a full paper-like density, provides good color reproducibility.
The current invention uses gelatin for the water-soluble resin of the top coat. The gelatin is easily combinable with cross-linkable additives (such as aldehydes, acids, urea resins and aluminum salts). Pigments may also be added, if desired.
The aforesaid chemistries have produced good results, but at a relatively high cost. The present invention seeks to achieve the same or better result at a lower cost.
In U.S. Pat. No. 4,887,097 (issued to Akiya et al on Dec. 12, 1989, for "RECORDING MEDIUM AND INKJET RECORDING PROCESS EMPLOYING THE SAME"), a printing medium is illustrated.
Although the aforesaid Akiya et al patent teaches using gelatin in combination with the resinous layers, it is not suggested for the top coat. In addition, the use of gelatin poses certain problems not addressed therein. These problems have been solved by this invention and are considered a vital part thereof.
The current invention uses low-cost, photographic-type gelatin as the ink-receiving layer of a printing medium. The gelatin utilized by this invention generally requires a bloom strength of approximately greater than 200 (and preferably between 200 and 250).
From a practical and commercial standpoint, gelatins cannot be used as the primary, or ink-retaining, layer of a film article, because gelatins dry out rapidly and lose their ink-retaining properties, strength and glassy finish. In fact, their shelf life is so poor that gelatins are all but useless as a stand-alone ink-receiving layer for glycol-type inks.
The present invention reflects the discovery that the addition of an amide (preferably, dicyandiamide) to the gelatin can improve its shelf life dramatically. The gelatin treated with an amide will retain its moisture over long periods of time. As a result, the invention allows the manufacture of a good ink jet film article, using gelatin as the top coat.